1. Field of the Invention
The disclosure relates generally to floating offshore structures. More particularly, the disclosure relates to buoyant semi-submersible offshore platforms for offshore drilling and production. Still more particular, the disclosure relates to the geometry of the hull and pontoons of semi-submersible offshore platforms.
2. Background of the Technology
Most conventional semi-submersible offshore platforms comprises a hull that has sufficient buoyancy to support a work platform above the water surface, as well as rigid and/or flexible piping or risers extending from the work platform to the seafloor, where one or more drilling or well sites are located. The hull typically comprises a plurality of horizontal pontoons that support a plurality of vertically upstanding columns, which in turn support the work platform above the surface of the water. In general, the size of the pontoons and the number of columns are governed by the size and weight of the work platform and associated payload to be supported. The draft of an offshore structure generally refers to the vertical distance between the waterline and the bottom of the structure.
Conventional shallow draft semi-submersible offshore platforms are used primarily in offshore locations where water depth exceeds about 300 feet (91 meters). A typical shallow draft semi-submersible platform has a draft between 60 ft and 100 ft. (18.3 m and 30.5 m), and incorporates a conventional catenary chain-link spread-mooring arrangement to maintain its position over the well site. The motions of these types of semi-submersible platforms are usually relatively large, and accordingly, they require the use of “catenary” risers (either flexible or rigid) extending from the seafloor to the work platform, and the heavy wellhead equipment is typically installed on the sea-floor, rather than on the work platform. The risers have a catenary shape to absorb the large heave (vertical motions) and horizontal motions of the structure. Due to their large motions, conventional semi-submersible platforms usually do not support high-pressure, top-tensioned risers.
Increasing the draft of a semi-submersible offshore platform can improve its stability and reduce its range of movement. Doing so involves lengthening the columns and locating the pontoons at a greater depth below the surface of the water, where wave excitation forces are generally lower. As a result, a deep draft semi-submersible offshore platform (i.e., having a draft of at least about 150 feet (about 45 m)) usually has significantly smaller vertical and rotational motions than a conventional shallow draft semi-submersible platform, thereby enabling the deep draft platform to support top-tensioned drilling and production risers without the need for disconnecting the risers during severe storms. In addition, the surface area of the upper and lower surfaces of the pontoons can be increased, resulting in the vessel having a greater added mass, and hence, increased resistance to movement through the water and heave natural period. With increased heave natural period, the peak wave energy can be avoided.
In both conventional and deep draft types of semi-submersible offshore platforms, the hull is divided into several closed compartments, each compartment having a buoyancy that can be adjusted for purposes of flotation and trim. Typically, a pumping system pumps ballast water into and out of the compartments to adjust their buoyancy. The compartments are typically defined by horizontal and/or vertical bulkheads in the pontoons and columns. Normally, the compartments of the pontoon and the lower compartments of the columns are filled with water ballast when the platform is in its operational configuration, and the upper compartments of the columns provide buoyancy for the platform.
The location of final assembly of a semi-submersible offshore platform may involve integration of the hull (i.e., the pontoons and columns) and work platform (topside) at the shipyard (quayside), offshore at its operation site, or nearshore (integration site). For integration at the shipyard, the work platform is lifted and mounted to the hull with heavy lifting equipment (e.g., heavy lift crane), and then the fully assembled semi-submersible platform is transported to the operation site using a heavy lift or tow vessel. This approach may not be possible for deep draft semisubmersible platforms that have relatively long columns. For integration at the operation site, the hull is transported offshore to its operation site, either by towing it at a shallow draft, or by floating it aboard a heavy lift vessel. When the hull is at the operation site, it is ballasted down by pumping sea water into the pontoons and columns, and the work platform is then either lifted onto the tops of the columns by heavy lift cranes carried aboard a heavy lift barge, or by floating the work platform over the top of the partially submerged hull using a deck barge. In either case, the procedure is typically effected far offshore (e.g., 100 miles, or 161 km), is performed in open seas, and is strongly dependant on weather conditions and the availability of a heavy lift barge, making it both risky and expensive. For nearshore integration, the work platform is lifted and mounted to the hull with heavy lift cranes or heavy lift barge in the water close to the shore, and then the assembled platform is transported to the operation site. As compared to assembly at the operation site, nearshore assembly is generally less expensive and less risky. However, as the water is generally shallower proximal the shore, nearshore integration may not be possible for some deep draft semi-submersible structures due to the length of the columns—due to water depth, the hull may not be capable of being ballasted down far enough to allow mounting of the work platform to the hull with a heavy lifting crane or heavy lift barge.
During drilling or production operations, it is generally desirable to minimize the motion of the offshore platform to maintain the position of the platform over the well site and to reduce the likelihood of damage to the risers. One component of offshore platform motion is heave, which is the vertical linear displacement of the offshore platform in response to wave motion. For use in conjunction with top tensioned risers or dry tree solutions, the floating structure preferably has heave characteristics such that the strokes (relative motion between the hull and the buoyancy can or risers) and the tension of the risers are within acceptable limits. Further, for use in conjunction with steel centenary risers or wet tree solutions, the floating structure preferably has heave characteristics such that the riser fatigue and strength requirements are within acceptable limits.
For most semi-submersible floating structures, heave is governed by the draft of the structure and the geometry of the hull. As previously described, in general, the deeper the draft of the structure, the less heave. However, increasing the draft of the hull may inhibit the ability to employ quayside topside integration. Further, increasing the draft of the hull usually results in increased hull weight, as well as increased materials and manufacturing costs.
Accordingly, there remains a need in the art for a semi-submersible offshore platforms with acceptable heave characteristics in lower draft applications, and which can be manufactured more cost effectively.